The present invention relates to an image compression technology, and more particularly to a technology for compressing an image into JPEG format.
In recent years, digital cameras have become increasingly sophisticated, for instance, by providing a huge number of pixels and offering a continuous shooting mode. It is therefore demanded that a high-speed process be performed to compress picked-up images.
A JPEG compression method for compressing image data to a predefined number of bytes at once is disclosed in Japanese Unexamined Patent Publication No. 2003-199019. This method integrates a high-frequency component of image data to obtain an integrated value, calculates, in accordance with the obtained integrated value, the number of bytes resulting when the image data is compressed, and calculates a quantization scale for compressing the image data to the predefined number of bytes at once. Next, this method creates a quantization table, which is used to compress the image data in accordance with the calculated quantization scale, and then compresses the image data with the created quantization table (claim 1).
The quantization scale is also referred to as the Q scale or Q value. It is used to calculate the quantization table (e.g., Q table or DQT), which is used when image data is compressed into JPEG format. It ranges in value from 1 to 100. The smaller the Q scale, the higher the compression ratio.
Further, when the image data derived from predetermined image signal processing is to be compressed, the method disclosed in Japanese Unexamined Patent Publication No. 2003-199019 integrates the high-frequency component of the processed image data (claim 4).